Field
The subject disclosure relates generally to the testing of subsurface well integrity. More particularly, the subject disclosure relates to sonic logging methods and apparatus for analyzing the condition of the cement about the casing(s) of single or double-cased wells.
State of the Art
In order to produce hydrocarbons from a geological formation, it is common to drill one or more boreholes in the formation and to install metal tubes (e.g., steel casing) that are cemented into place into each borehole. In some situations, a single casing is cemented in place in the borehole. In other situations, concentric casings are cemented in place, with cement between the casings and cement between the outer casing and the borehole. Holes in the casing and cement are created in order to establish a fluid connection between the reservoir and a producing wellbore. It is generally desirable that the cement fill entirely the space between the formation and the casing (and between the concentric casings where such an arrangement is used), as otherwise, different layers of the formation can be in uncontrolled fluid communication with each other via the borehole outside the casing. The functionality of the cement in preventing fluid communication between different formation layers, which is based on the structural integrity of the cement, is often referred to as “well integrity.”
Well integrity can be compromised because of a variety of cement defects ranging from debonding at an interface between the steel casing and cement or between the cement and the formation, to the presence of fluid channels in the cement annuli, to defects resulting from mud contamination of the cement slurry during the curing phase and from eccentered casings. Debonding at interfaces occurs because of several mechanisms including differences in the thermal expansion coefficients of the steel, cement and formation and cooling of the cement annulus as it cures, as well as casing expansion and contraction due to production-induced pressure changes within the inner casing. It may also be due to mud left on the casing or mudcake on the formation that prevents the cement from properly sealing to the casing and to the formation, respectively. Any presence of fluid channels in the cement weakens the cement integrity and can be a potential source of unwanted fluid communication between a reservoir and cap rocks.
One tool used for measuring formation characteristics is a sonic tool. The sonic tool may be used in a fluid-filled uncased borehole or in a fluid-filled cased wellbore. The sonic tool uses an acoustic source that generates head waves as well as relatively stronger borehole-guided modes in the fluid-filled borehole or well. More particularly, a sonic tool including a piezoelectric source and an array of hydrophone receivers is placed inside a fluid-filled borehole. The piezoelectric source is configured in the form of either a monopole or a dipole source. The source bandwidth typically ranges from a 0.5 to 20 kHz. A monopole source generates primarily a lowest-order axisymmetric mode, also referred to as the Stoneley mode, together with compressional and shear head waves. In contrast, a dipole source primarily excites the lowest-order flexural borehole mode together with compressional and shear head waves. The head waves are caused by the coupling of the transmitted acoustic energy to plane waves in the formation that propagate along the borehole axis. An incident compressional wave in the borehole fluid produces critically refracted compressional waves in the formation. Those refracted along the borehole surface are known as compressional head waves. The critical incidence angle θi=sin−1(Vf/Vc), where Vf is the compressional wave speed in the borehole fluid, and Vc is the compressional wave speed in the formation. As the compressional head wave travels along the interface, it radiates energy back into the fluid that can be detected by hydrophone receivers placed in the fluid-filled borehole. In “fast” formations, the shear head wave can be similarly excited by a compressional wave at the critical incidence angle θi=sin−1(Vf/Vs), where Vs is the shear wave speed in the formation. It is noted that head waves are excited only when the wavelength of the incident wave is smaller than the borehole diameter so that the boundary can be effectively treated as a planar interface. In a homogeneous and isotropic model of fast formations, compressional and shear head waves can be generated by a monopole source placed in a fluid-filled borehole for determining the formation compressional and shear wave speeds. It is known that refracted shear head waves cannot be detected in “slow” formations (where the shear wave velocity is less than the borehole-fluid compressional velocity) with receivers placed in the borehole fluid. In slow formations, formation shear velocities are obtained from the low-frequency asymptote of flexural dispersion.
Standard processing techniques have been developed for the estimation of formation shear velocities in either fast or slow formations from an array of recorded dipole waveforms. One of the techniques is known as the Slowness-Time-Coherence (STC) processing algorithm which estimates a non-dispersive slowness of an arrival from an array of waveforms over a chosen frequency filter and sampling window. Another technique uses variations of Prony's algorithm that isolates both dispersive and non-dispersive arrivals in a recorded wave train. Sonic logging provides measurements of non-dispersive and dispersive arrivals that can be analyzed to estimate elastic properties of the propagating medium.
These measurements in cased holes encounter additional challenges because of the presence of the steel casing bonded to the cement annulus. The steel casing is a strong waveguide and its associated modes interact with the formation modes. Interaction of the steel casing modes with those of the formation modes is strongly dependent on the mechanical properties of the cement annulus as well as the bond quality between the steel casing and formation.